Every steam or gas turbine installed in the world has a system to protect it from a destructive overspeed event. If the turbine speed reaches a certain level, different for each machine depending on its design, major mechanical failure will occur. Although there are many different designs and configurations for the overspeed trip protection system, every one of them is designed to sense when the shaft speed reaches a predetermined speed limit and then activate a shutdown system.
The simplest and most common protection systems consist of a mechanical device mounted on the shaft that moves when the speed limit is reached. The mechanical device then strikes a stationary mechanism that is linked to a valve. The interaction of the overspeed device and the mechanical linkage results in the rapid closing of a steam or gas valve.
Systems can range from the simplest mechanical designs, as described above, to very sophisticated electronic detection and valve actuation systems with very rapid response times. Whether simple or complex, however, the systems need to be tested from time to time to verify proper operation.
In addition to simply verifying that the system functions, it is also important to measure the response of the system and document all of the results. When a machine is equipped with an electronic control system and that control system is integrated into an overall computerized plant control system (DCS) then it is possible to perform an overspeed test and have the speed history during the test documented. However, the DCS only stores speed versus time and does not measure or record anything else.
There is no system known to this writer which is designed to allow the operator to measure anything more than speed when they are testing a turbine trip system. The “state of the art” for conducting overspeed trip system tests is to visually monitor speed and either watch or listen for the sudden closing of the stop valve. The operator must visually and mentally associate a speed value with the moment the trip system activates. This is true even for a system that records speed as described above.
It is therefore an object of the present invention to allow routine testing and documentation of turbine overspeed trip systems that includes measurement and recording of all the system components: shaft speed, the activation of the overspeed sensing device and the movement of the stop valve or valves.